Exhaust gas recirculation (EGR) systems recirculate a portion of exhaust gas from an engine exhaust to an engine intake system to improve fuel economy and vehicle emissions by reducing throttling losses and combustion temperatures. In turbo-charged direct injection engines, a low-pressure EGR (LP-EGR) circuit may be implemented. The LP-EGR circuit recirculates exhaust gases from an exhaust passage downstream of a turbine to an intake passage upstream of a turbocharger compressor.
In order to provide EGR over a wide-range of operating conditions, LP-EGR systems may utilize a specialized EGR schedule. One example EGR schedule is shown by Fujita et al. in US 20070246028. Therein, a fixed amount of EGR is delivered at all EGR conditions, the EGR amount delivered as one or more of low pressure EGR and high pressure EGR. Another example schedule is shown by Styles et al. in US 20120023937. Therein, LP-EGR is delivered at a fixed EGR rate (or percentage) relative to fresh airflow at all engine loads except high loads.
However, the inventors herein have identified potential issues with such schedules. As an example, delivering EGR as per the schedules of Styles or Fujita may lead to combustion instability and engine misfire events during transient operations due to the presence of excess EGR dilution. This is largely due to the pre-compressor location of EGR delivery. Particularly, in turbo-charged systems, providing EGR though the LP-EGR circuit may cause long transport delays as the exhaust gases have to travel though the turbocharger compressor, high-pressure air induction plumbing, charge air cooler, and intake manifold before reaching the combustion chamber. As a result of the transport delay, during conditions when EGR needs to be rapidly reduced, such as during a tip-out operation where the engine goes from a high load and high EGR rate condition to a low load and low EGR rate condition, EGR may not be purged from the air intake system fast enough. As a result, there may be elevated intake-air EGR dilution during the low load condition until the EGR is purged from the air intake system. The presence of increased intake-air dilution at low loads can increase combustion stability issues and the propensity for engine misfires.
While the flat schedule of Styles may reduce the likelihood of high EGR amounts at lower engine loads, the schedule may also limit the fuel economy benefits of LP-EGR. For example, the flat EGR schedule may result in LP-EGR being provided at some low load points where no fuel economy benefits from EGR are achieved. In some cases, there may even be a fuel penalty associated with the delivery of LP-EGR at the low load point. As another example, the lower EGR at the lower load points may limit the peak EGR rates achievable during subsequent higher load engine operation. The delayed purging of EGR requiring EGR in the engine intake system at low engine loads can also render the intake compressor susceptible to corrosion and condensation. Furthermore, increased condensation may occur at the charge air cooler of a boosted engine system due to the flow of EGR through the cooler. The increased condensation may necessitate additional counter-condensation measures which further reduce engine efficiency and fuel economy.
In one example, some of the above issues can be at least partly addressed by a method for an engine comprising: operating an engine with all cylinders combusting while flowing low pressure EGR; and responsive to decreasing engine load, disabling EGR and operating the engine with one or more cylinders deactivated until EGR is below a threshold. In this way, the EGR tolerance of the engine at low loads is improved.
As an example, during medium to high engine loads, an engine may be operating with low pressure EGR (LP-EGR) flowing to provide fuel economy and emissions benefits. The LP-EGR flow may be adjusted to be at a fixed rate relative to airflow. In response to a decrease in engine load to low load conditions, such as due to an operator pedal tip-out, air flow may be decreased (e.g., by adjusting an intake throttle) while also decreasing LP-EGR flow (e.g., adjusting an LP-EGR valve). For example, an EGR valve may be adjusted to provide lower engine dilution at the lower load conditions. However, the purging of the EGR from the engine system may occur slower than desired due to transport delays in the LP-EGR system. Specifically, more EGR may be present in the air induction system, specifically at a pre-compressor location, than desired, degrading combustion stability and potentially inducing misfires.
In order to improve the low load engine combustion stability and EGR tolerance, in response to the decreasing engine load, while EGR is reduced, one or more cylinders may be selectively deactivated. For example, fuel to the one or more cylinders may be cut off while intake and exhaust valves are deactivated. A number of cylinders deactivated may be based on the decrease in engine load. In one example, the engine may have two banks of cylinders and in response to the decreasing engine load, all cylinders of a first engine bank may be deactivated while all cylinders of the second bank are maintained active. As a result, for the same engine torque, the cylinders of the second bank may be working at a higher average cylinder load. The higher load operation of the active cylinders allows for engine operation at the lingering EGR with reduced likelihood of misfires and slow burn issues. In addition, by isolating the air volume of the deactivated bank, the effective boosted volume of the engine is decreased, and LP-EGR depletion is expedited. The engine may continue to be operated with one or more cylinders deactivated until LP-EGR has been depleted to a threshold level. In response to a subsequent tip-in, the previously deactivated engine cylinders may be reactivated. In addition, during the reactivation, fueling may be adjusted so as to purge an exhaust catalyst coupled to the deactivated group of cylinders. As such, the fuel economy gain from the higher cylinder load operation and expedited EGR purging may balance or outweigh the fuel economy associated with the purging of the exhaust catalyst.
In this way, EGR purging from an engine intake can be expedited. By selectively deactivating one or more engine cylinders during decreasing engine load and decreasing EGR conditions, the average cylinder load can be increased, improving cylinder EGR tolerance and combustion stability. By isolating the air volume of the deactivated cylinders, the effective boosted volume of the engine is decreased and LP EGR depletion is expedited. As such, this enables EGR levels in the air induction system and intake manifold to be reduced faster (e.g., up to half the time) than would have been otherwise possible. By rapidly reducing the intake EGR level at low load conditions and by increasing EGR dilution tolerance at these low engine load conditions via cylinder deactivation, higher EGR rates can be achieved when the engine is subsequently restarted. As such, this substantially improves engine efficiency, particularly in medium to high engine speed-load regions. By replacing the EGR with fresh air, evaporation of water and hydrocarbon condensates is increased, reducing their concentration in the engine, and the need for counter-condensation measures. In addition, the reduction in condensation reduces compressor and charge air cooler corrosion and degradation. Overall, boosted engine performance is improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.